This new mental ray material with an ambient/reflective occlusion map needs to be applied on to any other object in the scene which is not using an opacity map. Once this is done, hit render. Your render should look something like what is shown below in Render01.

Render 1

In this first instance the scene will look very noisy, because the sample rate under the ambient/reflective occlusion map is default at 16, so change this value to around 128 to make your scene less noisy, as shown below (Render02).

Render 2

The whole ideology for me showing this tutorial was imagining that have a huge scene with some 100s of materials applied to the objects, out of which 20 materials are applied to the plants or trees, or any other object that has opacity to this. Using this method you can simply apply one ambient/reflective occlusion map to all the materials with opacity, and make a new mental ray material for the rest of the objects not with any sort of opacity. In one of my personal scenes I had a really hard time getting ambient occlusion with alpha. Using all the different plug-ins gave me lots of problems! However, in the end I found this procedure to be very stable, even in the bigger scenes, without any errors or crashes! I hope this will be a great help to you guys also in the creation of your own scenes.